1. Field of the Invention
This invention relates to methods of processing a substrate, and more specifically, to a method for the removal of metal contaminants during the cleaning process of, for example, semiconductor wafers.
2. Prior Art
In the manufacture of semiconductor devices, the surface of semiconductor wafers must be cleaned of wafer contaminants. If not removed, wafer contaminants may affect device performance characteristics and may cause device failure to occur at faster rates than usual.
In general, there are two types of wafer contamination: particulates and films. Particulates are any bits of material present on a wafer surface that have readily definable boundaries, for example, silicon dust, atmospheric dust, plastic particles, and silicate particles. Films are layers of foreign material on wafer surfaces, for example, metallic films, photoresist residues, and solvent residues. It should be noted that films may break loose and become particles, for example, surface metals.
As there are two types of wafer contamination there are also separate cleaning procedures to remove each of them. Film contaminants are generally removed by chemical cleaning processes and particulates are generally removed by ultrasonic scrubbing or a combination of high pressure spraying and mechanical scrubbing.
The most commonly used chemical cleaning process is the RCA method. The RCA method is used for cleaning bare silicon or silicon wafers which only have thermally grown oxides. This process consists of six steps. First, there is a preliminary cleaning of the wafer. The wafer is immersed in a sulfuric acid-oxidant mixture (such as H.sub.2 SO.sub.4 --H.sub.2 O.sub.2) which removes the photoresist present on the wafer. Often, even if the resist has previously been stripped, the wafer is immersed into the above described sulfuric acid-oxidant mixture to prepare the wafer surface for the next step in the process. When the wafers are removed from the sulfuric-oxidant mixture they are then rinsed in 18-23.degree. C. deionized and filtered water. Such water is also used for all other rinse steps of the RCA method.
The second step of the RCA method is the removal of residual organic contaminants and certain metals. A fresh mixture of water, ammonium hydroxide, and hydrogen peroxide (H.sub.2 O--NH.sub.4 OH--H.sub.2 O.sub.2, 5:1:1 by volume) is prepared and heated to approximately 75-80.degree. C. The wafers are then submersed into the solution for approximately 10-15 minutes, while the temperature is maintained at approximately 80.degree. C. The wafers are then rinsed for approximately 1 minute in deionized water.
If a thermally grown silicon dioxide film does not completely cover the wafer, then the third step of the RCA method comprises stripping of the hydrous oxide film formed in step two. The wafers are submersed into a mixture of hydrofluoric acid and water (HF--H.sub.2 O, 1:10 by volume). Following immersion in the hydrofluoric acid, the wafers are rinsed, but only for 20-30 seconds. The short rinse minimizes regrowth of the oxide.
The fourth step of the RCA method is desorption of the remaining atomic and ionic contaminants. A fresh mixture of water, hydrochloric acid, and hydrogen peroxide (H.sub.2 O--HCl--H.sub.2 O.sub.2, 6:1:1 by volume) is prepared and heated to approximately 75-80.degree. C. The wafers are submerged into the solution for 10-15 minutes and are then rinsed in deionized water.
Steps five and six comprise drying and storing the wafers, respectively. It should be noted that in all the steps previous to drying, the wafers are kept wet in between each of the steps.
The RCA method is widely used but still has some problems. One problem with the RCA method is that vapors of ammonia and hydrochloric acid form a particulate smoke of ammonium chloride (NH.sub.4 Cl) when intermixed. Thus, the solutions of step two and step four should be kept separate (i.e. under different exhaust hoods) to avoid wafer contamination from colloidal NH.sub.4 Cl particles. A second problem with the RCA method is that the cleaning solution of step two must be closely monitored to prevent depletion of H.sub.2 O.sub.2, as NH.sub.4 OH in the absence of H.sub.2 O.sub.2 will etch silicon. As an example, depletion of H.sub.2 O.sub.2 may occur if the solution temperature is allowed to rise above 80.degree. C., at which point rapid decomposition of H.sub.2 O.sub.2 ensues. Another example is, if impurities are allowed to accumulate in the solution they may accelerate the decomposition of H.sub.2 O.sub.2. Another problem with the RCA method is that almost every step concludes with a deionized water rinse. Thus, the process as a whole consumes a large amount water. Also, a further problem with the RCA method is that additional metal contaminants are actually deposited on the wafer surface due to the ammonium hydroxide.
An improvement to the RCA method is the use of centrifugal spray cleaning rather than immersion cleaning in solutions. The process of centrifugal spray cleaning operates by using a sequence of fine sprays of cleaning solutions and high purity water to wet the wafers, which are enclosed in a chamber purged with N.sub.2. There are many advantages to using centrifugal spray cleaning instead of immersion cleaning in solutions, for example: smaller volumes of chemicals and deionized water are consumed (approximately two thirds less) and wafer surfaces are continually exposed to fresh reagent solutions. Another benefit of centrifugal spray cleaning is that since the wafers do not have to be transferred from one solution to another the system may be automated thus allowing the environment of the process to be carefully controlled.
One method for removal of insoluble particulate contamination is Ultrasonic Scrubbing. In ultrasonic scrubbing wafers are immersed in a suitable liquid medium to which sonic energy in the range of 20,000 to 50,000 Hz is applied. Cavitation, the rapid forming and collapsing of microscopic bubbles in the liquid medium under the pressure of sonic agitation, produces shock waves which impinge on wafer surfaces. The shock waves displace and loosen particulate matter. An improvement upon ultrasonic scrubbing is the megasonic bath. The megasonic bath incorporates the same principles as ultrasonic scrubbing, however, the megasonic bath uses higher frequency sonic waves, approximately 850 kHz. Also, the megasonic bath may be operated with the same solutions used in the RCA chemical film removal process. The use of the RCA solutions in the megasonic bath allows chemical cleaning and contaminant desorption while simultaneously removing particulates.
As with all cleaning processes, ultrasonic scrubbing and the megasonic bath have problems associated with them. One problem, is the need to prevent shock waves from carrying the particles and redepositing them on the wafer surfaces once they become detached and fall into the liquid medium. Currently, to remedy this problem particles are removed through overflow or filtration. Another problem associated with ultrasonic scrubbing and the megasonic bath is mechanical failure of the substrate film as a result of the ultrasonic energy imparted during the cleaning cycle. Frequently, mechanical failure of the substrate film results in film loss in certain regions, or in the extreme, the entire film may be removed. A further problem with ultrasonic scrubbing and the megasonic bath is that more metals may be deposited on the substrate surface than are removed, if the liquid medium contains ammonium hydroxide.
Another method for the removal of insoluble particulate contamination is the combination of High Pressure Spraying and Brush Scrubbing (brush scrubbing). There are two types of brush scrubbing for silicon wafers: scrubbing one side of a wafer and scrubbing both sides of a wafer, both work in substantially the same manner. During scrubbing a brush is rotated across the surface of the wafer. The brush does not actually touch the wafer, but hydroplanes over its surface. The brush (or brushes) impart motion to the solvents of the scrubber and the moving solvent dislodges particulates. High pressure jet spraying is almost always used with brush scrubbing. The high pressure jet sprays deionized water, at a pressure of approximately 13.8-20.7 MPa, across the surface of the wafer and removes the particulates dislodged by the brush as well as any residual particles generated by the brush.
The common problem to all the above mentioned wafer cleaning methods is that none of them remove metals from the wafer surface. The RCA method, due to the use of ammonium hydroxide, tends to add more metals to the wafer surface rather than remove them. Ultrasonic scrubbing, the megasonic bath and brush scrubbing have the same problem as the RCA method if the scrubber solvent includes ammonium hydroxide. Another problem that ultrasonic scrubbing, the megasonic bath and brush scrubbing have in common is that they do not remove metals unless they are in the form of particulates, and even then they have the problem of redepositing the metals back onto the wafer surface.
What is needed is a method to remove metal contaminants from wafer surfaces that may replace or be used in combination with existing wafer cleaning methods for each type of contaminant.